The chromogranin/secretogranins (or "granins") are a family of regulated secretory proteins ubiquitously found in the cores of amine and peptide hormone and neurotransmitter secretory vesicles. Evidence has now been gathered in support of both intracellular and extracellular functions for this protein family. Within the cells of origin, a granulogenic or sorting role in the regulated pathway of hormone or neurotransmitter secretion has been documented, especially for its index member, chromogranin A (CgA). Granins also function as pro-hormones, giving rise by proteolytic processing to a large array of peptide fragments for which diverse autocrine, paracrine, and endocrine activities have been demonstrated in vitro, ex vivo and more recently in vivo. For example, CgA fragments vasostatin and catestatin control vasoreactivity and catecholamine release, and the CgA fragment pancreastatin elevates blood glucose. This proposal develops 2 specific aims, employing novel granins chimeric proteins, to investigate in situ, mechanisms whereby chromogranin (CgA) is sorted into catecholamine storage granule of sympathoadrenal chromaffin cells, and interacts with other secreted or membrane proteins of the secretory granule. The responses to be investigated are exocytosis and FRET (fluorescence resonance energy transfer) molecular communication resulting from protein-protein interaction. In aim 1, we will use a series of CgA domains tagged with green fluorescent protein (GFP) or embryonic alkaline phosphatase (EAP), to identify in situ, the putative sorting signals within the primary structure of CgA (cis determinant) that mediate CgA's targeting to the chromaffin granule. In aim 2, granin chimeric proteins will be used to investigate in the living cell, which determinants of the secretory apparatus (trans determinants) interact with CgA to influence sorting of the granin into the regulated secretory pathway and its storage within the core of the catecholamine storage granule. The results of these studies will enhance our knowledge of catecholamine/granins storage and release during sympathetic stimulation, leading to a better understanding of neuroendocrine secretory granule biogenesis .